1. Field of the Invention
This invention relates generally to scissors lifts, and more particularly to such lifts adapted for use in repetitively raising and lowering items of furniture, home entertainment divices, office equipment, and other such articles. Some preferred embodiments of the invention repetitively raise and lower such articles in such a way as to provide access to the article when it is in a raised position and concealment when it is in a lowered position.
Although the invention is by no means limited to domestic or office usages, for convenience in this document it is sometimes referred to as a cabinetry lift.
2. Prior Art
(a) Scissors Lifts: General History--Many ingenious people have developed ways to use scissors mechanisms to raise or extend platforms, baskets, and scaffolds carrying various sorts of payweights. In particular, several patents have addressed the problems encountered in initiating the extension of a scissors mechanism from a fully retracted or folded position. These patents will be identified below, and the reason for the initial-extension problem will be discussed.
As will be seen, however, none of these patents has dealt with the detailed behavior of a scissors mechanism at the opposite end of its operating range--that is to say, in its extended position--or even in the midregion between the extended and retracted positions. In the prior art, an extended scissors mechanism is retracted simply by removing, reducing or even reversing the primary driving force: the mechanism readily starts down. Moreover, the apparatuses used for application of external driving force to a scissors mechanism generally accommodate a relatively wide variation of resistance from the scissors mechanism; they simply pump in more energy. Thus, once the problems that occur near the retracted position have been solved, there has been no need to be concerned with the magnitude of the lifting force at the other end of the operating range.
(b) Tension-extended Scissors Systems--Perhaps the "first generation" of scissors lifts is typified by U.S. Pat. Nos. 1,078,759 and 1,817,418. The first of these issued in 1913 to Arend Wichertjes, and the second in 1931 to Arthur Munns. Both disclose multiple-stage scissors lifts--or, as they are sometimes called, "lazy tong" mechanisms. These are scissors lifts in which one "scissors" linkage drives another above it, which in turn may drive yet others.
Wichertjes and Munns respectively describe chain-controlled and cable-controlled scissors lifts. In each case the chains or cables are wrapped around the lateral pivots (and across the central pivots) of the successive scissors linkages. When tensioned, the chains or cables pull the lateral pivots together to extend the lift.
Wichertjes notes that "it might result in undue stress and strain upon the lazy-tongs to rely upon the chains . . . alone for extending the device and elevating the platform," and accordingly he provides an "auxiliary elevating device". The "stress and strain" to which Wichertjes alludes apparently arise from the fact that when a force that is purely lateral, or almost purely lateral, is applied to open or extend a fully folded or retracted scissors mechanism, there is a strong tendency for the mechanism to bind rather than to extend. When this happens, if the forces applied are increased the result is often to break something rather than to extend the mechanism.
The binding can be understood by studying the mechanism. The forces on the rigid members are directed almost exactly within and parallel to the lengths of those members, with at most a very small component of force directed perpendicular to the rigid members to rotate them about their pivot points. Often the "rigid" members of a loaded scissors mechanism that is fully folded are slightly deformed (bent or twisted) by the load, causing the rotational-tending force to be actually zero. Sometimes these forces are even caused to be applied in a direction that tends to rotate the arms to a more tightly folded position. Only when the scissors is partly open does there develop a sizable component of force directed to rotation in the proper direction and thereby to further extension.
Though Wichertjes does not say so, the tendency to bind is actually a special case--or an extreme manifestation--of the strongly varying mechanical advantage which a scissors mechanism presents to its driving force. When the driving force is applied to pull the ends of the legs at one end of the scissors straight toward each other, the mechanical advantage between the driving force and the weight to be moved at the far end of the scissors varies as the tangent of the angle between the legs and (for a vertical scissors) the horizontal. When the scissors mechanism is fully folded this angle is very nearly zero, the tangent and thus the mechanical advantage are likewise, and only a tiny fraction of any input force is therefore available to open the scissors (the rest, as already observed, being applied to break something).
Wichertjes resolves this impasse by providing a completely separate chain-drive mechanism for raising part of the scissors linkage vertically, in preparation for operating his main mechanism to extend the scissors by pulling its opposite pivot points together as previously described. Wichertjes' entire device generally is disadvantageous by virtue of being almost startlingly complicated or elaborate, and seemingly impractical by virtue of this intricacy.
Munns also directs his attention to the initial-extension problem, but he ascribes it (somewhat inaccurately, it would appear) to inadequate available "power"--rather than to the tendency to bind. He observes, "The mechanisms heretofore proposed for moving the lazy-tongs to extended position from a folded position have been such as to render very difficult the initial actuation thereof to the extent of requiring a relatively greater source of power and one wholly beyond the range of practicability particularly where the elevator is a portable one and great loads are adapted to be lifted." He adds that "although the pulley and cable mechanism thus far described is sufficient to move the lazy-tong structure when dealing with light loads, it is incapable of initiating movement of the lazy-tong structure when elevating relatively heavy loads."
Although Munns' text at some points appears inaccurate as to the problem which he is trying to solve, his text at other points is quite accurate as to the means applied to solve it: "the pulling force which may be said to be acting horizontally is . . . converted into a vertical force which operates to move the arms upward." By referring to "force" rather than "power", Munns here correctly focuses on the previously described adverse behavior of the mechanical advantage of a scissors mechanism at small angles. Whereas ample power may be available, the scissors mechanism misdirects the available force.
Munns' conversion redirects the line of action of the available force so that it can perform the desired work. Munns effects this conversion by separate members fixed to two of the scissors arms and extending a substantial distance downward from them, and pulley wheels at the lower ends of these arms; the cables crossing the bottom scissors stage are passed under these two pulley wheels, causing each cable to assume a "V" shape and thus creating a large vertical component of tension. This tension tends to raise the wheels, and operates the mechanism out of the range of positions in which binding is a serious problem--whereupon the primary mechanism takes over. Munns' device suffers from the severe disadvantage that his downward-extended extension members are very awkward or cumbersome, and in particular prevent collapsing the mechanism to a very shallow configuration.
Even after the Wichertjes or Munns mechanism has been elevated past the point at which binding is a serious problem, the adverse (that is, very low) mechanical advantage at small angles continues to require relatively large force levels for extension of the mechanism. Notwithstanding Munns' above-quoted comments, such force levels generally can be obtained through gearing. Nevertheless, the requirement of large forces can be a particularly severe problem if these forces must be borne by cables or chains in tension, since very strong (and therefore large-diameter and heavy) cables or chains are thereby required, and the apparatus as a whole must be very large, bulky, heavy, and expensive. The weight and expense of the necessary gearing further aggravates these factors.
Hence the auxiliary lifting arrangements of the Wichertjes and Munns devices are used to move the mechanisms not only out of the dead zone in which the scissors actually bind, but also past the range of positions in which the mechanical advantage is so unfavorable that (1) the driving force would be stalled, and/or (2) excessively heavy-duty force-transmitting elements would be required. It is emphasized that these devices of the prior art both operate by externally supplied energy, of which--in the past--the availability of an ample amount has generally been assumed. The auxiliary devices described merely serve to optimize the coupling of this externally supplied energy to drive the scissors.
Once the scissors legs in these mechanisms have moved a few degrees from the vertical, however, the auxiliary mechanisms are no longer needed. Even if stopped, the scissors can then be driven upward by the primary driving-energy source provided. In particular, neither Munns nor Wichertjes is concerned with reversing the direction of the mechanism from the fully extended position, since reversal is easily accomplished by removing, reducing or reversing the force applied to the driven end of the scissors.
A "second generation" of innovations in scissors mechanisms is offered by U.S. Pat. No. 4,391,345, which issued to Jim Paul on July 5, 1983. This patent discloses a much smaller, simpler, and more sophisticated approach to supplying the vertical component of force necessary to initiate extension of a cable-driven three-stage scissors mechanism.
Paul's device uses an eccentrically pivoted sheave a few inches in diameter, mounted to the scissors mechanism near the bottom. The sheave is readily rotated by the tension in the driving cable. It acts as a cam, raising the scissors legs through a few degrees of rotation and thereby past the region of very adverse mechanical advantage.
Paul suggests that the abandonment of cable-driven-scissors devices earlier in the century, in favor of hydraulic-cylinder-driven scissors devices, may have been due to the complex, cumbersome character of auxiliary apparatus used for the initial extension by inventors such as Wichertjes and Munns. Paul goes on to propose that his simpler and more compact initial-extension unit restores the cable-driven scissors to the realm of competitive practicality, since hydraulic systems are by comparison very heavy and expensive to operate.
However this may be in the field of large, vehicle-mounted, multiple-stage platform lifts, cable-driven systems are distinctly disadvantageous in the area of cabinetry lifts intended for high-volume manufacture and for final assembly in homes and offices by mechanically unskilled users or relatively unspecialized technicians. Cable-driven systems are characterized by a relatively large amount of manufacturing labor and inventory costs, because of the numerous small parts (particularly pulleys) that are involved. They also require a relatively large amount of final assembly work, and this work requires some level of specialized skill because of the necessity to thread the cables correctly and ensure that there are no snags. In addition cable-driven scissors lifts tend to be slow and rather noisy.
Nevertheless the principle of Paul's invention appears in modern devices, such as the line of electrically powered and cable-driven scissors lifts marketed by the firm Hafele America under the trade name "Open Sesame electric hideaway lift systems".
The Paul patent and the principles of the Hafele apparatus, like the earlier units previously discussed, are unconcerned with the details of operation of the scissors in the extended position. The purpose of the auxiliary devices in all these units is to facilitate operation near the retracted position of the scissors.
(c) Compression-extended Scissors Mechanisms--Preceding and paralleling Paul's innovation is the development of scissors mechanisms that are self-extending, driven by hydraulic cylinders or by electrical motors and screws. Generally at least one stage of the scissors mechanism in such devices is driven by pushing or pulling the legs together at one end, as in the cable-driven devices discussed previously; consequently the comments offered earlier regarding the tangent variation of mechanical advantage apply to these apparatuses as well.
U.S. Pat. No. 2,471,901 to William Ross, issued May 31, 1949, discloses one such system. Ross's apparatus provides a tiltable platform, one end being supported by a two-stage scissors. (It is a full or true scissors to the extent that it raises both stages vertically, though the upper or second stage is only a partial scissors in the sense that it does not hold the platform horizontal.) The other end is supported by an extension linkage that does not hold itself vertical as does a scissors. Only the former of these two mechanisms, accordingly, is pertinent to the present discussion.
Ross provides two features to mitigate the adverse mechanical advantage of the scissors mechanism in its retracted condition. First, he applies driving force from his hydraulic cylinder to a forcing point that is offset from the driven leg of the scissors; this geometry provides some rotation-tending component of force even when the mechanism is fully retracted. Second, Ross provides a second hydraulic cylinder which is mounted for purely vertical motion, to raise the first stage of the scissors bodily out of the low-mechanical-advantage region.
The primary and auxiliary hydraulic cylinders are both driven by a hand-cranked oil pump, to raise the scissors and payweight.
First, as to the offset forcing points, Ross mentions that his primary hydraulic cylinder acts on "off-set torque-lugs", apparently to aid mechanical advantage near the fully retracted position. From his drawings it appears that each forcing point is spaced from the rotational axis of the bottom of the respective leg by nearly half (about 0.46) of the length of the leg, and is offset approximately seventeen degrees (about the rotational axis) from the respective leg. The magnitude of these values has certain significance, which will be discussed later.
Second, as in Paul's cable-extended device, the auxiliary driving mechanism of Ross's hydraulic system--namely, Ross's vertical auxiliary cylinder--is provided:
"owing to the dificulty encountered at the point of substantially zero lift when the carriage . . . is in its lowermost position . . . [W]hen the upward travel of the carriage is initiated, the two piston-rods]. . . aid the main cylinders and their piston-rods until the limits of travel of the former have been reached at which time the main hydraulic means will be in such angular relation as to be properly effective to complete the lifting movement of the carriage. PA1 "Stated somewhat otherwise, the primary use of these `booster` or supplementary, upright, hydraulic means is to aid the `breaking` or starting of the upward motion of the pantograph-linkages . . . ."
Thus the auxiliary device is not intended to serve any function relating to operation in the extended position of the scissors.
Furthermore, when the apparatus is to be lowered from the extended position, this function "is accomplished in the usual manner by means of release valves of conventional design . . . ." In other words, the primary driving force is removed, and the weight on the platform lowers the scissors.
Moreover, also paralleling the cable-driven scissors disclosures, Ross's hydraulic unit deals with the variation of mechanical advantage in the midrange and extended positions of the scissors simply by supplying the varying force required to support the payweight.
Another patent in this area is U.S. Pat. No. 3,750,846, which issued Aug. 7, 1973, to Thomas Huxley. This patent discloses a multistage scissors that is driven either by an electric motor in combination with a screw or by a hydraulic cylinder. The first stage of the scissors in Huxley's device is not driven by pulling or pushing the legs together, but rather by pushing straight outwardly on the center pivot of the first stage. Nevertheless, the first stage necessarily extends the second stage by pulling the legs of the second stage together, so the previously discussed problems of mechanical-advantage variation are not completely eliminated. Due to play in the mechanism, the tendency for the outer stages to bind is as serious in Huxley's device as in those of Wichertjes and Munns.
Huxley responds to this difficulty by providing a separate device for boosting the last stage of the scissors out of its retracted or folded condition. This device is a spring which is compressed by a small part of the travel of the last stage during retraction--that is, just the last fifth or fourth of the travel. The spring stores the compression energy, and is sufficient to carry the full load of the payweight basket; it tends to drive the last stage out of the fully retracted condition. This tendency, however, is offset by the retracted condition of the adjacent stages of the scissors.
The tendency to extend the last stage, however, is used when the time comes to extend the entire mechanism. In effect, as Huxley explains, "Unfolding forces . . . commence at opposite ends of the boom structure and work towards the center . . . greatly facilitating the successive opening of the crossed links beyond critical angles . . . " The critical angles of which Huxley speaks arise, apparently, from distortion of the individual links, rather than from driving geometry.
Like the patents previously discussed, Huxley's is concerned with unfolding of his scissors mechanism from its fully retracted condition. Inspection of Huxley's disclosure reveals no passage directed to the detailed operation of the mechanism when it is extended
(d) Scissors Mechanisms: Other Factors--The Munns, Paul and Huxley patents represent a "second generation" of developments in the scissors-mechanism lift field. They are directed to producing optimum performance in terms of reliability and convenience.
Modern users of equipment, however, demand more than this. The present age is extremely conscious of the usage of energy, particularly nonrenewable energy sources. The modern age is also extremely conscious of the usage of materials, particularly metals, and of hand labor. It is furthermore extremely conscious of the usage of space, since the per-square-foot cost of usable home, office, and even light industrial space has skyrocketed in the last decade. Even the weight of equipment itself can be a critical factor, since shipping cost and ease of installation depend on this characteristic.
It has therefore become a matter of paramount concern to all manufacturers, and certainly to manufacturers of lifts intended for high-volume manufacture and for use in expensive home and business square footage, that apparatus be efficient in terms of labor, energy usage, space, materials, and shipping weight--while the equipment remains just as reliable and convenient as before.
Perhaps less plain, but equally significant in terms of energy and materials efficiency, is the undesirability of making several different models of lifts for use with articles of different weights--or, in other words, for different "payweights". It is desirable to standardize as much of a lift mechanism as possible, leaving a bare minimum of different submodules that must be changed to accomodate different payweights.
The use of different payweights arises from the infinitely various types of articles which end-users may wish to see repetitively raised and lowered. Thus it is neither possible nor particularly desirable to eliminate nonuniformity of payweights in use.
Yet there are many inefficiencies in the practice of manufacturing substantially different lifts for different payweights. Such inefficiencies extend through warehousing, spare-parts maintenance, billing and bookkeeping systems, and communications complexity all along the distribution chain from manufacturer to user.
(e) Energy-recycling Systems: General Introduction--In another field, the field of mechanical energy-storage devices, certain basic developments have arisen which have never been used in scissors lifts. It is not clear whether it has ever previously occurred to anyone skilled in the art of lift mechanisms to attempt to provide a scissors mechanism in combination with an energy-storage device, to recycle the energy released in lowering a payweight for the purpose of raising the same payweight subsequently.
One special kind of energy-storing lift that has been developed is a vertically acting cable-counterweighted lift, similar to an elevator or dumb waiter. This type of device does not involve a scissors mechanism. The energy in this type of device is stored as potential energy of height of the counterweight. Such devices, as previously noted for cable-driven scissors lifts, are disadvantageous by virtue of the need for several pulleys and the need to thread cables correctly. The resulting cost and labor requirement makes such devices undesirable in comparison to a scissors lift.
Thus the energy-storage approach has distinct appeal for use in scissors lifts.
(f) Energy-recycling Systems: Springs--One basic energy-storage device is of course the common mechanical spring. Springs are used in a wide variety of applications to "balance" various kinds of objects that are repetitively moved: the general goal is for the spring generally to support the object, while relatively small forces are supplied externally to move the object.
As is familiar to almost everyone in modern society, this goal is only marginally reached. The most common example is the spring suspension of horizontally pivoted (that is, vertically acting) doors, and particularly garage doors. The pervasive commercial success of automatic openers for garage doors is, in part, testimony to the incomplete ability of springs to balance large, heavy objects throughout their complete operating range.
The reason for this limitation apparently resides in the typical force-versus-travel characteristic of a spring: the force varies quite steeply with displacement (as a fraction of spring length) from the at-rest position of the spring. Suspension of a heavy object through a long displacement consequently requires use of a very long spring (so that the displacement can be made a relatively small fraction of the spring length). Thus garage-door suspension springs, despite clever use of mechanical linkages to minimize the necessary spring displacement, are typically three or four feet long.
Another disadvantage of springs is that if they break or lose their anchorage and whip around--or even if they are used with inadequate planning for unexpected release of the spring-driven mechanism--they can cause severe damage or injury. Garage-door suspension springs are at least favorably positionable on the opposite side of the door from the person moving the door, but this advantage is not available in many applications where it might be desirable to install lifts.
These limitations are particularly salient in the field of cabinetry lifts for indoor use, since space is at a distinct premium and it is difficult to arrange a single spring with sufficient travel to suspend a heavy object. The limitations of springs are also salient in this same field, and in the broader field of repetitively acting lifts, since in these fields it is typical for valuable and relatively fragile objects to be positioned--and for personnel to work--near the mechanism on a regular basis.
It is undoubtedly for these reasons that energy-recycling scissors lifts using springs are unknown. Even linearly, vertically acting lifts or jacks relying upon springs to recycle energy are not in common use, although they have been in the patent literature for many years. U.S. Pat. Nos. 727,192 (issued May 5, 1903 to Olen Payne) and 3,007,676 (issued Nov. 7, 1961 to Laszlo Javorik) each describe a vehicle jack with a spring that is compressed beforehand, storing energy for use in raising a vehicle. Mere brief speculation on the workings and typical uses (and users) of such articles suffices to explain their commercial nonexistence.
(g) Energy-Recycling Systems: Gas Cylinders--A recent innovation commercially is the permanently sealed gas cylinder, which contains a fixed quantity of gas (subject to very slight leakage, over a service period of several years) and which exerts an outward force on a piston. These gas cylinders are to be clearly distinguished from the earlier and better-known pneumatic and hydraulic cylinders that must be connected through valving to pressure sources--such as compressors, compressed-gas tanks, or pumps (as in the Ross patent).
An interesting aspect of these devices is that the force-versus-travel characteristic can be, and almost always is, made extremely shallow. In fact, the force is usually made very nearly independent of varying position of the piston, over the operating range of the apparatus in which the cylinder is installed. In this way practically constant force is made available for the purposes of the apparatus. A manufacturer of these gas cylinders is the West German firm Suspa-Federungstechnik GmbH, of Altdorf.
Each cylinder contains a small amount of oil, in addition to the driving gas, for the purpose of lubricating the action of the piston in the cylinder--and also for the purpose of controlling the speed at which the piston reacts to changes in adjustment or externally applied forces.
These cylinders have been used in such applications as supporting automobile hatchbacks and controlling office-chair seat heights. As can be readily understood, the shallow force-versus-travel characteristic of the devices is quite useful in such units. In some units for use in office chairs, the force-versus-travel curve for these devices is modified by changing the amount of oil, or in other ways, to superpose a relatively steeply rising segment at short cylinder extensions. Doing this provides a cushioning effect as users of the chairs sit down.
If it ever previously occurred to anyone to use such cylinders in connection with cabinetry lifts generally or with scissors lifts in particular, the idea would very likely be dismissed out of hand, for reasons to be set forth in the discussion of the invention.
(h) Summary: The foregoing comments show that there has been a need in the cabinetry-lift industry for a third generation of scissors lifts, one that is (1) substantially more compact, simpler in construction, and lighter in shipping weight than those of the second generation but (2) at least as convenient and reliable, and (3) capable of accommodating any payweight with minimal change of components. This need arises from considerations of energy, labor and materials efficiency, and efficiency in general, and also from considerations of reliability in use.
These comments also show that the concept of (4) recycling the energy used in repetitive raising and lowering of the payweight has some tantalizing benefits for the scissors-lift industry, but that this concept has never been applied to scissors lifts.